High-pressure single-crystal X-ray diffraction study of katoite hydrogarnet: Evidence for a phase transition from Ia3d → I4̄3d symmetry at 5 GPa

The crystal structure of katoite hydrogarnet has been refined at 0.0001, 2.15, 4.21, 5.09, 6.00, 7.09, and 7.78 GPa from single-crystal X-ray diffraction data using a 4:1 methanol: ethanol mixture as pressure medium in a Merrill-Bassett diamond-anvil cell. Below ~5 GPa, the katoite structure has la3d symmetry and compresses by bond shortening rather than bond bending, in agreement with recent quantum mechanical calculations. An unconstrained third-order Birch-Murnaghan fit to the unit-cell volumes and pressures for la3d symmetry gave the following equation of state parameters: V_o = 1987.6(1) ų, K_o = 58(1) GPa and K' = 4.0(7). Above this pressure, the structure undergoes a phase transition to space group I43d, a non-centric subgroup of la3d. In the I43d structure. there are two non-equivalent (O₄H₄) groups. Both the Ca and Al atoms are displaced along a relative to their positions in la3d. It is proposed that compression of the short H-H distance between (O₄H₄) groups destabilizes the structure and may initiate the observed phase transition. Corroboration of this model will require accurate information on the hydrogen atom positions at pressures above 5 GPa.